Currently, look for 12.1, 12.0, 11.4 and 11.2. (But of course I recommend
people keep their Xcode updated, and thus now build against the 12.1
SDK.)

Also get rid of two AC_SUBST variables that were not used anywhere.

Change-Id: Ic1abf3eb1d900c843263dc781fd4c9a8ab126f09
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>